If downloading and installing the game isn't an issue for you, I'd recommend logging in (it's free, after all), going to the first/main city, sitting down for a while and checking just how things are ; ) They pretty much changed the main city from Fedimian to Klaipeda (this being literally the second map you'll visit) so that new players could check on the game activity.

...on the other hand, Steamcharts isn't that reliable of a source, as -I assume- it counts the total players instead of players per server. From what I've seen, for instance, Silute has a lot more players (and bugs, and technical problems, and...) than the sever I play in, Orsha.

Generally speaking, though, I'd say the game is still going strong; while I do appreciate the effort in keeping Tosbase (my first step for ToS-related stuff), the site has fallen behind more recent and updated content. But if you take a look at the official English ToS site (https://treeofsavior.com/): there's an active forum community there and you'll be able to check on updates and game previews. The upcoming Rank 9 and 4 new classes are to be implemented in kToS (the Korean server) this week, whereas these updates are still to be announced for iToS (the Steam ToS servers). On the upside of things, this means there's still some considerable content and future plans for the game: at least the developers aren't planning on letting the game die anytime soon.